Mule 4 and Munit 2.1.5 java.lang.ClassNotFoundException for customer assertion

I have a customer assertion class which is located under src/test/java or /src/test/munit either 

If I run the munit, I get  

java.lang.ClassNotFoundException: com.example.CustomAssertion

the definition of the customer assertion is taken from the documentation 

https://docs.mulesoft.com/munit/2.1/run-custom-event-processor

the munit test file

 <munit:validation >

<munit-tools:assert-that doc:name="Assert the return of MessageID " doc:id="61e7edf9-fa90-4228-8794-5ff31c20cc6c" expression="#payload" is="#MunitTools::notNullValue()" message="MessageID should not be null or empty"/>

<!-- customer assertion -->

<munit-tools:run-custom assertion="com.example.CustomAssertion" expression="#payload" />

</munit:validation>

The documentation does not state any additional details.

Any idea why this class is not found even it is on the test class path